I. Field of the Invention
The present invention relates generally to dry powder compositions for forming abradable coatings, methods for making the compositions, and methods for coating articles with the compositions, and the coated articles themselves, such as pump components.
II. Description of the Related Art
In fluid transferring devices, e.g., pumps or compressors, the operation relies on the relative movement of internal components to pump liquids or compress gases. For the highest operating efficiency of such devices, it is necessary to minimize leakage between the movable components without having contact between the components. In some devices it is also advantageous to minimize friction between the movable components and/or between the components and the fluid being transferred. Abradable coatings on the working surfaces of the pumps grind themselves into nearly perfect fitting surfaces when the pump is first operated. These conventional abradable coatings are made of liquid materials which are sprayed or painted onto the working surfaces. Alternatively, the working surfaces can be dipped into the liquid abradable coating composition. The liquid coating may even be electrostatically painted on.
However, these prior art liquid compositions have inherent problems, including cost for excessive material, environmental concerns arising from the use of the solvents, and the inability to recycle and re-use any overspray which is surely generated during any spraying or electrostatic applications. Dipping and roller painting may not generate much overspray, but they certainly generate a lot of toxic solvent fumes.
One example of the prior art is U.S. Pat. No. 5,554,020 to Rao et al. which discloses providing a liquid abradable coating on both contacting surfaces in a gas compressor. Before start up, the coated contacting surfaces have an interfering fit; but upon start up, the coatings on the two contacting surfaces abrade and grind against each other to a substantially zero clearance.
Rao et al. use either water-based or solvent-based resin formulations to coat the desired components and subsequently flash off the solvent or water and cure the resin. Rao et al.'s formulations include the water or evaporative solvent along with certain solid lubricants, a thermoset resin selected from epoxy, polyamide, or polyaryl sulphone, and a polymerizing catalyst. Rao et al. describe that the coating may be applied by (i) electrostatic or air atomized spray/or dip process or (ii) a smooth sponge roller. In the case of a spray process, a multi-layer coating is taught to be desirable.
Although Rao et al. present the above-described methods for achieving close clearances on pump components, their methods are not without problems and undesirably high manufacturing costs. For instance, when solvent-based formulations are used, removal, containment, and special handling of the solvent is required. Removal of the solvent (or volatile organic compound) from the coatings—which is completed before the curing process—requires additional heat, time, and handling equipment. The volatile organic compound requires additional care in handling for the safety and health of the operators and the environment. In addition, there is typically significant shutdown time necessary for cleaning and maintenance of solvent-processing equipment. Aside from the problems with processing with solvents, solvent-based materials also create unique problems and additional cost when transporting the materials.
Although the water-based systems are environmentally more favorable, there remains the additional cost and time of evaporating and handling the water from the coatings and the shutdown time for cleaning and maintenance of the equipment. Furthermore, not all resins can be formulated into water-based systems, so the types of resins available to use with water-based systems are limited.
Another disadvantage of spray coating liquid-based. formulations is that it is not practical to recycle any overspray. Reclaimed overspray would require an inordinate amount of re-formulation to adjust its viscosity in order to achieve consistent coating results.
Therefore, even in light of the Rao et al. patent, there is a need for improved abradable coatings, methods of making the coatings, methods of coating articles with abradable coatings, and the coated articles themselves. It would be advantageous if the improved coatings were easy to apply, cost effective, energy-wise, used relatively inexpensive and simple equipment, and were environmentally favorable. It would also be advantageous if the composition for forming the abradable coatings was recyclable, to reduce loss during the coating process. Yet another advantage would be realized if any coating thickness can be achieved in one layer, thus, not requiring a multi-layer coating. Still a further advantage would be realized if the lubricity of the coating could be controlled to best meet different applications.
Still further advantages would be realized if an improved abradable coating were made available, e.g., one which is more easily abraded and/or one in which less material is required, thereby saving cost.